This invention relates, in general, to large free-standing steam generators of the type used in combined cycle power plants; and, in particular, this invention is a control system for controlling the input of feedwater into a boiler drum.
In a combined cycle power plant, a heat recovery steam generator (HRSG) is the connecting thermal link between a gas turbine and a steam turbine. The HRSG channels hot gas turbine exhaust gas passed finned heat exchanger tubes to hear steam/water contained in the tubes in a counterflow heat exchange relation. A boiler drum is associated with the heat exchanger tubes and normally produces steam for a superheater.
Boiler controls could be designed to maintain a fixed drum level set point. One type of control is a single-element feedwater control. Single-element control will maintain a constant drum level for slow changes in load, steam pressure, or feedwater pressure. However, since the control signal satisfies the requirements of drum level only, excessive "swell or shrink" effects will result in wider drum-level variations and a longer time for restoring drum level to set point following a load change. The foregoing single-element feedwater control can be improved by a two-element control comprising a feed forward control loop which utilizes steam flow measurement to control feedwater input, with level measurement assuring correct drum level. Once again, the drum level correction must be slow-acting because there are times, particularly during startup when there is no steam flow.
A three-element control system is a cascaded-feed forward control loop which maintains water-flow input equal to feedwater demand. This type of control system is fully explained in a latter portion of the present discourse and the present invention seeks to improve upon the function of the system. The improvement relates to a response time and reliability in a three-element control system.